The accurate measurement of temperature is vital across a broad spectrum of human activities, including materials processing (e.g. making steel), manufacturing (e.g. parts only fit perfectly at a certain temperature), food (preparation, transport and storage), health, and of course scientific discovery. In fact, in almost every sector, temperature is one of the key parameters to be measured.
One difference between temperature and other physical properties, such as mass or length, is that it is intensive. If we consider two objects with the same mass and temperature, then their combined mass is the sum of the masses of the individual objects. However, their combined temperature will be unchanged. So although it is fairly easy to imagine ways in which we can determine how much heavier one object is than a standard mass, it is not at all obvious how to determine how much hotter one thing is than a 'standard temperature'.
The 'standard temperature' we use is the temperature of the triple point of water, which is the unique temperature at which the three phases of water (solid, liquid and vapour) co-exist in equilibrium. We define this temperature to be 273.16 K exactly and hence determine the size of the unit of temperature to be:
The fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
From this single temperature, it is possible to work out the temperatures of other highly reproducible 'fixed points' such as melting temperature of pure gallium metal (302.9146 K) or the freezing temperature of gold (1337.33 K). Such experiments are not easy and are rarely done, but once the temperatures of the fixed points have been established they were incorporated into the International Temperature Scale of 1990 so that all types of thermometers can be conveniently calibrated with excellent reproducibility.
The degree Celsius (°C) has the same size as the kelvin, but is offset from kelvin scale by 273.15 to make it conveniently similar to the historical unit the degree centigrade, which is no longer used.
Temperature affects a wide variety of physical processes because all substances are composed of atoms, and fundamentally temperature is a measure of the average energy of the motion of the atoms within an object. From May 2019, it is expected that the kelvin will be defined in terms of this microscopic motion, and a new definition will be based on a fundamental constant known as the 'Boltzmann constant' that measures how much energy of motion corresponds to one kelvin.
One advantage of this definition is that any fixed point could be used as a standard temperature, and any appropriate method for temperature measurement could be used. This allows for the possibility of improved uncertainty of temperature measurement at extremely high and extremely low temperatures.